A Human Interactive Proof (HIP), also known as CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) differentiates a human user from automatic programming (i.e., a bot).
The goal of most HIP schemes is to prevent automated access by a computer, while allowing access by a human. Typically, this goal is addressed by providing a method for generating and grading tests that most people can easily pass, and that most computer programs cannot pass.